Visual imperfections while rendering with scattering atmosphere
The problem with the 'red' scattering is due to how you are using the Volume Scatter shader - this should not generally be used on its own and should typically be combined with a Volume Absorption shader, otherwise it will producce unrealistic effects.

The 'scatter' node produces the result of the scattering in a volume whereas the 'absorption' node attenuates the light as it passes through to emulate losses due to actual absorption and also the losses due to scattering. Without combining the absorption with a Density at least as high as that used for scattering you will be breaking the usual laws of conservation of energy - effectively generating light from nowhere.

You should combine the absorption as follows :



The example is produced by two spheres - one with a diffuse shader and another (slightly larger) sphere with the volumetric material shown. Note that the density is much smaller than in your example - and ensure the volumetric 'Step' size is small enough to produce enough steps through the volume of your atmosphere.

See also https://blender.stackexchange.com/a/92327/29586 for a more realistic atmosphere (one that falls off realistically in relation to altitude).